This invention relates to thermoelectric material and, more particularly, to high-efficiency thermoelectric material, a process for producing thereof and Peltier module using the thermoelectric material.
The figure of merit Z is convenient for evaluating the thermoelectric material, and is expressed as follows.
Z=xcex12/(xcfx81xc3x97xcexa)xe2x80x83xe2x80x83equation 1 
where xcex1 is the Seebeck coefficient in xcexcxc2x7V/K, xcfx81 is the electric resistivity in xcexa9xc2x7m, xcexa is the thermal conductivity in W/mxc2x7K. The greater the figure of merit is, the more preferable the thermoelectric material is. From equation 1, it is desirable for the thermoelectric material to have a small electric resistivity and a small thermal conductivity. In general, it is known to persons skilled in the art that the thermal conductivity is reduced together with the grain size. It is also the known fact that the electric resistivity is reduced together with the number of crystal grains through which the electric current flows. Thus, the figure of merit is improved by controlling the growth of crystal.
One of the crystal structure controlling technologies is carried out by using a hot pressing. A sintered body is a typical example of the solidified thermoelectric material in Bi2Te3 system. A thermoelectric element is made from the thermoelectric material as follows. The thermoelectric material is pulverized, and the resultant powder is shaped into a sintered product through a hot pressing. While the powder is being sintered in the hot pressing, the crystal grains tend to be solidified in such a manner that a-axes of the crystal grains, which are the low-resistive direction of the crystal, are oriented in the perpendicular direction to the direction of the pressure. When the electric current flows in the low-resistive direction, the sintered product exhibits a large figure of merit. For this reason, the manufacturer spaces electrodes in the low-resistive direction on a piece of sintered product. The electric current flows through the crystal grains in the direction parallel to the a-axes, and the piece of the sintered product exhibits a large figure of merit. The piece of sintered product is used as an essential part of the thermoelectric element, and plural thermoelectric elements are assembled into a thermoelectric module.
Another crystal structure controlling technology is disclosed in Japanese Patent Application laid-open No. 11-163422. The crystal structure controlling technology disclosed in the Japanese Patent Application laid-open No. 11-163422 is carried out through an extrusion. FIGS. 1A and 1B show the prior art extrusion process. The prior art extrusion process starts with preparation of a bulk 101 of thermoelectric material as shown in FIG. 1A. The thermoelectric material has the composition containing at least one element selected from the group consisting of Bi and Sb and another element selected from the group consisting of Te and Se.
A die unit 102 is heated with a heater 104, and the bulk 101 of the thermoelectric material is pressed to the die unit 102 as indicated by an arrow in FIG. 1B. The bulk 101 is softened, and a rod 103 of the thermoelectric material is extruded from the die unit 102. While the soft thermoelectric material is passing through the die unit 102, the soft thermoelectric material is subjected to the slit orientation, and a large amount of crystal grains are oriented so as to have (001) crystal plane, i.e., c-plane in a certain direction. After the extrusion, the thermoelectric material forming the rod 103 is solidified to have fine crystal grains without changing the orientation. Although the electric resistivity xcfx81 is not varied between the bulk 101 and the rod 103, the thermal conductivity xcexa is lowered.
Yet another crystal structure controlling technology is disclosed in the Proceedings of 2000 Spring Conference of Japan Society of Powder and Powder Metallurgy. According to the proceedings, a bulk of thermoelectric material is forced to pass through an elbow passage. The bulk is pressed against the inner surface, and a sharing force is exerted on the bulk of thermoelectric material for orienting the crystal grains.
FIG. 2 shows an extruder used in the prior art crystal structure controlling technique disclosed in the proceedings. Reference numeral 110 designates the die unit 110, and a passage 110a is formed in the die unit 110. The passage 110a has an elbow-like shape. A green compact 112 is formed from powder of p-type thermoelectric material expressed as (Bi2Te3)0.2 (Sb2Te3)0.8. The green compact 112 is put into the passage 110a, and a punch 111 is inserted into the passage 110a. The punch 111 presses the green compact 112 against the inner surface of the die unit 110, and a sharing force is exerted on the green compact 112. The green compact 112 is bent, and a plate 113 of the thermoelectric material is extruded from the die unit 110. While the sharing force is being exerted on the green compact 112, the crystal planes are oriented in a certain direction.
Still another crystal structure controlling technology is disclosed in Japanese Patent Application laid-open No. 178218. FIGS. 3A and 3B show the process of the hot upset forging disclosed in the Japanese Patent Application laid-open. The process starts with preparation of an ingot of solid solution of thermoelectric material. The ingot is pulverized, and the resultant powder is subjected to a pressure sintering.
The sintered product 124 is placed in an inner space of the upset forging machine as shown in FIG. 3A. The upset forging machine has a base plate 121 and column-shaped sleeves 122. The base plate 121 and the sleeves 122 are assembled together so as to define the rectangular parallelepiped inner space. A punch 123 is movable in the rectangular parallelepiped inner space.
The sintered product 124 is heated, and the punch 123 is downwardly moved. A compressive force is exerted on the sintered product 124. The plastic deformation takes place in the sintered product 124, and the sintered product 124 is stretched on the base plate 121 as shown in FIG. 3B. The crystal grains of the sintered product 124 are oriented in a direction at which the figure of merit is improved. Thus, the thermoelectric semiconductor material 125 is improved in the figure of merit through the hot upset forging.
A problem is encountered in the prior art crystal structure controlling technologies described with reference to FIGS. 1A, 1B, 2, 3A and 3B in that the products 103/113/125 are different in thermoelectric properties between the p-type thermoelectric material and the n-type thermoelectric material. In detail, it has been known to the persons skilled in the art that the p-type thermoelectric material is superior in thermoelectric properties to the n-type thermoelectric material. When the manufacturer designs the p-type thermoelectric material and the corresponding n-type thermoelectric material to have the Seebeck coefficient equal therebetween, the n-type thermoelectric material obtained through any one of the prior art crystal structure controlling technologies is higher in electric resistivity than the p-type thermoelectric material also obtained through the same prior art crystal structure controlling technology. If the manufacturer designs the p-type thermoelectric material and the corresponding n-type thermoelectric material to have the electric resistivity equal therebetween, the n-type thermoelectric material obtained through any one of the prior art crystal structure controlling technologies is higher in the Seebeck coefficient than the p-type thermoelectric material also obtained through the same prior art crystal structure controlling technology. In fact, the manufacturer thinks it impossible to produce n-type thermoelectric material in the (Bi, Sb)2 (Te, Se)3 system with the figure of merit greater than 3.00xc3x9710xe2x88x923/K through any one of the prior art crystal structure controlling technologies. However, the manufacturer requires the n-type thermoelectric material and the p-type thermoelectric material equal in thermoelectric properties to each other for a thermoelectric module. Especially, the thermoelectric module applicable to an optical communication system requires n-type thermoelectric material equal in electric resistivity to or less than 2xc3x9710xe2x88x925 xcexa9xc2x7m and greater in figure of merit than 3.0xc3x9710xe2x88x923/K. However, such a high-performance n-type thermoelectric material is not presently manufactured. Thus, the problem inherent in the prior art thermoelectric material is that both of the n-type thermoelectric material and the p-type thermoelectric material in (Bi, Se)(Te, Se) system can not achieve the figure or merit greater than 3.0xc3x9710xe2x88x923/K.
The prior art crystal structure controlling technology described with reference to FIGS. 1A and 1B has another problem in dispersion of properties and a low production yield. While the bulk 101 is moving from the wide space to the narrow space, the bulk 101 is squeezed to the rod 103 as shown in FIG. 4A. However, the bulk 101 is not uniformly squeezed. The peripheral portion 130 is strongly squeezed, but the central portion 131 is less squeezed. This phenomenon results in the low production yield. This means that the amount of crystal grains with (001) planes oriented in the certain direction is different between the peripheral portion 130 and the central portion 131. Since the thermal conductivity is dependent on the amount of crystal grains with (001) planes oriented in the certain direction, the central portion 131 is different in thermal conductivity from the central portion 131. If the manufacturer designs the thermal conductivity in the peripheral portion 130 to a target value, the central portion 131 is out of the target range, and, accordingly, is not used for the thermoelectric element. If the rod 103 is thin, only a small amount of the thermoelectric material is available for the thermoelectric element. Furthermore, while the rod 103 is being extruded from the die unit 102, the rod 103 is rotated in the die unit 102. The rotation of the rod 103 results in (001) crystal planes 132 arranged in the direction 133 of the rotation as shown in FIG. 4C. Although crystal grains are in the peripheral portion 130, the crystal grains exhibit different electric resistivity, and a part of the peripheral portion 130 is not available for the thermoelectric elements. Thus, the manufacturer suffers from a low production yield.
Another problem is further encountered in the prior art crystal structure controlling technology described with reference to FIGS. 3A and 3B in high electric resistivity. The ingot of solid solution of the thermoelectric material is pulverized into the powder before the pressure sintering. For this reason, the crystal grains of the sintered product 124 are relatively large and lack of uniformity. Even though the sintered product is subjected to the hot upset forging, the large and non-uniform crystal grains make the thermoelectric semiconductor 125 exhibit a large electric resistivity. In n-type thermoelectric semiconductor material, the large electric resistivity is serious.
It is therefore an important object of the present invention to provide thermoelectric material, which exhibits a large figure of merit regardless of the conductivity type thereof
It is also an important object of the present invention to provide a process for producing the thermoelectric material.
It is another important object of the present invention to provide a thermoelectric module using the thermoelectric material.
In accordance with one aspect of the present invention, there is provided a thermoelectric material composed of at least one element selected from the group consisting of Bi and Sb and at least one element selected from the group consisting of Te and Se, and comprising crystal grains having respective [001] directions and an average grain size equal to or less than 30 microns, certain crystal grains having the [001] directions crossing a direction in which an electric current flows at 45 degrees or less, said certain crystal grains occupying an area equal to or less than 10% on a section perpendicular to the direction.
In accordance with another aspect of the present invention, there is provided a process for producing a thermoelectric material composed of at least one element selected from the group consisting of Bi and Sb and at least one element selected from the group consisting of Te and Se, and the process comprises the steps of a) preparing a fusion of the thermoelectric material, b) rapidly solidifying the fusion so as to obtain flakes of the thermoelectric material, c) stacking the flakes so as to form a lamination, d) putting the lamination into a die having an inlet portion and an outlet portion obliquely extending with respect to the inlet portion and e) pressurizing the lamination for extruding a bulk of the thermoelectric material from the die unit at least once so that a sharing force is exerted on the lamination at a boundary between the inlet portion and the outlet portion.
In accordance with yet another aspect of the present invention, there is provided a process for producing a thermoelectric material composed of at least one element selected from the group consisting of Bi and Sb and at least one element selected from the group consisting of Te and Se, and the process comprises the steps of a) preparing one of an ingot of the thermoelectric material and a powder of the thermoelectric material, b) putting aforesaid one of the ingot and the powder into a die unit having an inlet portion and an outlet portion obliquely extending with respect to the inlet portion and c) pressurizing aforesaid one of the ingot and the powder for extruding a bulk of the thermoelectric material from the die unit at least once so that a shearing force is exerted on aforesaid one of the ingot and the powder at a boundary between the inlet portion and the outlet portion.
In accordance with still another aspect of the present invention, there is provided a thermoelectric module for producing a temperature difference from an electric current passing therethrough, and the thermoelectric module comprises a pair of substrates having respective inner surfaces opposite to each other, conductive layers formed on the inner surfaces and plural thermoelectric elements of a first conductivity type and other thermoelectric elements of a second conductivity type held in contact with the conductive layers so as to be alternately connected in series, each of the thermoelectric elements consists of the plural thermoelectric elements and the other thermoelectric elements including a piece of thermoelectric material and metal layers, the piece of thermoelectric material is composed of at least one element selected from the group consisting of Bi and Sb and at least one element selected from the group consisting of Te and Se, the piece of thermoelectric material comprises crystal grains having respective [001] directions and an average grain size equal to or less than 30 microns, certain crystal grains have the [001] directions crossing a direction in which an electric current flows at 45 degrees or less, and the certain crystal grains occupy an area equal to or less than 10% on a section perpendicular to the direction.